In order to carry out an efficient conversion of a spectrum of electromagnetic radiation into electrical current, devices called monolithic cascade converters have been developed which provide multiple, typically two semiconductor junctions within one device, one junction having a low characteristic electromagnetic energy gap and the other junction a high characteristic electromagnetic energy gap. The energies of the low and high energy gaps are chosen so as to optimize coverage of the incident electromagnetic spectrum, thereby maximizing the efficiency of converting incident electromagnetic radiation into electrical energy. For example, the theoretical efficiency for a two-junction device has been calculated to be in excess of 35% near ambient temperature for a typical solar spectrum measured at the earth's surface.
Currently, only group IIIa-Va semiconductor materials technology is sufficiently developed to produce reasonably efficient multiple junction devices. However, in the case of solar cell materials, a substantial portion of the materials and cost is accounted for by the necessity of using GaAs as a substrate material. Gallium metal is in high demand and in limited supply; therefore, the cost of GaAs substrate material is presently about $3.20/cm2 compared with approximately $0.30/cm2 for Si wafers. The end result is that the GaAs substrate represents approximately 25 percent of the total device cost. Further, the thermal conductivity of GaAs is only 0.54 W/cm.degree.C. compared with 1.41 W/cm.degree.C. for Si. In a solar concentrator, the efficiency of converting light to electricity can decrease dramatically if the semiconductor junction temperature increases too much. Therefore, since the thermal conductivity of Si is nearly three times that of GaAs, Si is much more desirable for use in a solar concentrator system than GaAs. However, because Si is not lattice matched to any of the appropriate high energy gap materials, Si has not been utilized as a substrate in an efficient monolithic cascade converter device. Growth of one material on top of another with a difference in lattice parameters between the layers (lattice mismatched) will result in lattice mismatch stresses. These stresses in turn lead to formation of line defects (threading dislocations) in the overlayer, which in the case of a two-junction device is the high energy gap layer. The end result is that these dislocations cause severe degradation of conversion efficiency in the high energy gap portion of the device. PG,4 It is therefore an object of the present invention to provide a multiple junction, monolithic cascade device for converting incident electromagnetic radiation into electrical energy.
It is also an object of the invention to provide a two-junction, monolithic cascade device for converting incident electromagnetic radiation into electrical energy.
It is also an object of the invention to provide a two-junction monolithic cascade device for conversion of a solar spectrum into electrical energy.
It is a further object of the invention to provide a two-junction, monolithic cascade device for conversion of the solar spectrum into electrical energy having Si as a substrate and low energy gap material.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.